1. Field of the Invention
The present invention relates to an electronic device and method for connecting a die to a connection terminal. In particular, the invention regards a ribbon-bonding method.
2. Discussion of the Related Art
As is known, in the technical field of packaging of integrated electronic circuits, there is felt the need to connect electrically dies and connection terminals (also known as posts) by means of bonds that introduce low values of resistance, and withstand, without melting, the passage of high currents, in the region of 150-200 A. This need is particularly felt in the case of so-called power packages, which are commonly used for packaging integrated circuits that form devices with high current consumption, such as, for example, power diodes, low-voltage power MOSFETs, or insulated-gate bipolar transistors (IGBTs).
As is known, in order to bond a die to a connection terminal, commonly used today are the so-called wire-bonding, clip-bonding, and ribbon-bonding techniques.
By way of example, FIG. 1 is a schematic illustration of a package 1, which is formed by a die-pad area 2 of metal material and having the function of carrying a die 4, which provides a mechanical support to the die 4 itself; in particular, the die 4 is connected to the die-pad area 2 by means of an appropriate bonding paste, and is protected by a protective case (not shown), made of insulating material and forming part of the package 1. In addition, the package 1 comprises three leads 6, which enable connection of the die 4 to the outside world, and form, together with the die-pad area 2, the so-called lead frame 8. The package 1 further comprises a tab 10, which is made of metal material, is connected to the die-pad area 2, and defines a fixing hole 12.
In greater detail, each of the leads 6 has a respective connection terminal 14. By way of example, the connection terminals of two leads 6 set externally have a so-called “T” shape, so that they are generally known as “T-posts”.
Operatively, to enable proper operation of the die 4 and of the electronic circuits implemented therein, it is necessary to bond the die 4 itself with one or more of the leads 6. For this purpose, it is possible to resort, amongst other techniques, to the aforementioned wire bonds, clip bonds, or ribbon bonds.
As regards wire bonding, it is likewise possible to distinguish between thin-wire bonding and heavy-wire bonding; in either case, the bonding is carried out by means of an element of conductive material, having a circular section.
In the case of clip bonding, the bond is carried out by means of a so-called clip, i.e., a strip of conductive material (typically, copper), so as to obtain a quasi-planar bonding.
As regards, instead, ribbon bonding, bonding between the die 4 and at least one of the leads 6 is carried out using a sort of ribbon of conductive material, typically aluminum.
By way of example, FIG. 2 shows an example of ribbon bonding, inside an electronic device 15 formed by a die, designated once again by 4, and a respective package, designated once again by 1. Further elements, already shown in FIG. 1 and present also in the electronic device 15 shown in FIG. 2 are designated by the same reference numbers. For reasons of clarity, the protective case of the die 4 is not shown in FIG. 2 either.
In particular, the electronic device 15 comprises a first connection terminal 14a, a second connection terminal 14b, and a third connection terminal 14c, which respectively form part of a first connector 6a, a second connector 6b, and a third connector 6c; in addition, the electronic device 15 comprises a ribbon 16, made for example of aluminum, or else of aluminum-cladded copper.
In detail, the ribbon 16 is electrically connected to the first connection terminal 14a and to the die 4. In particular, the die 4 has a body 17, made of semiconductor material, and a metallization 18, which is set in contact with the body 17 and defines a top surface 20 of the die 4 itself. In addition, the ribbon 16 is electrically connected to the metallization 18 of the die 4, while a bottom surface 22 of the die 4, opposite to the top surface 20, is connected to the die-pad area 2 by means of a layer of bonding paste (not shown). In practice, the metallization 18, made, for example, of aluminum, defines the so-called front die, which is electrically connected to the first connection terminal 14a by means of the ribbon 16.
In greater detail, the ribbon 16 has a certain flexibility and has a first contact surface 24a, a second contact surface 24b, and a third contact surface 24c, which are to a first approximation the same as one another and substantially rectangular in shape. In particular, the first contact surface 24a is formed by the portion of the ribbon 16 effectively in contact with the first connection terminal 14a, while the second and third contact surfaces 24b, 24c are formed by portions of the ribbon 16 effectively in contact with the metallization 18 of the die 4.
The electronic device 15 likewise comprises a wire bond 26, which electrically connects the third connection terminal 14c to the die 4. In particular, the metallization 18 is shaped so that the wire bond 26 is electrically connected not to the metallization 18, but rather to the body 17 of the die 4. The second connection terminal 14b is, instead, formed integrally with the die-pad area 2, as well as with the tab 10.
Purely by way of example, the die 4 can house a power MOSFET 27, and the first, second, and third connection terminals 14a-14c can be electrically connected, respectively, to the source, drain, and gate terminals of the power MOSFET 27; in this case, the metallization 18 coincides electrically with the source terminal, whilst the die-pad area 2 coincides electrically with the drain terminal.
FIG. 3 shows a cross section of the ribbon 16, which has a rectangular shape and a width L and a thickness T. The thickness T is negligible with respect to the length (not shown) and to the width L of the ribbon 16; in particular, the thickness T is not greater than one third of the width L.
In general, the ribbon bonds are characterized by low resistances, and enable supply to the dies, or rather to the electronic circuits formed therein, of particularly high currents, hence representing a very flexible form of connection suitable also in the case of so-called power devices.
In greater detail, the ribbons designed to form the ribbon bonds are generally obtained by means of rolling of a conductive material (aluminum, cladded copper, etc.) into sheets having a thickness T; next, the sheets are cut in such a way that the ribbons thus obtained have the desired widths L. From a more quantitative standpoint, the ribbons available today typically have thicknesses T and widths L that are in a ratio with respect to one another of one to ten, or else of one to eight. In absolute terms, the maximum thickness Tmax and the maximum width Lmax of a generic ribbon are, respectively, equal to 10×80 mils, i.e., 0.254×2.032 mm.
In practice, in the technical field of packages of integrated electronic circuits, the ribbon bonds are obtained using ribbons the thickness T and width L of which are substantially standardized, according to the machines currently available for creating the bonds themselves, generally known as bonding machines. In particular, the aforementioned maximum width Lmax is set by the bonding machines currently available for providing wire bonds. In fact, it is common practice to use the same bonding machines to carry out both wire bonding and ribbon bonding; however, this is possible provided that the ribbons have widths not greater than the maximum width Lmax; otherwise, the bonding machines known today do not enable handling of ribbons in an appropriate way.
In detail, a generic bonding machine comprises, amongst other things, one or more guides, a bonding tool and a transducer. In practice, the guides are such as to bring a wire, in the case of wire bonds, or else a ribbon, in the case of ribbon bonds, in the proximity of at least one die, carried by a respective die-pad area. When the wire, or else the ribbon, is in the proximity of the aforementioned die, the connection tool exerts a pressure on the wire/ribbon, in such a way that it contacts the die, and in particular the metallization that forms the front die. In addition, the transducer transmits ultrasounds, which impinge on the portion of wire/ribbon in contact with the metallization of the die; in this way, by means of pressure and exposure to ultrasounds, there is partial melting of the conductive material that forms the metallization and the wire/ribbon, with consequent formation of the bond.
As has been mentioned, the ribbon bonds enable supply to the dies of particularly high currents; for example, a generic ribbon having a thickness T and a width L respectively equal to ten and eighty mils is able to withstand the same total current that can be conveyed by four round wires with a diameter of fifteen mils. Consequently, the ribbon bonds enable simplification of the electrical connections between dies and connection terminals. However, the ribbon bonds, and in particular the geometrical dimensions of the ribbons, should respect the geometrical constraints imposed by the bonding machines, with consequent limitation of the benefits that can be achieved adopting said type of bonding.